In a radio communication system, a plurality of adjacent channels are usually used for communication. Accordingly, for preventing or decreasing power leakage between these adjacent channels, it is important to suppress nonlinear distortion in the power amplifier mounted in the system by linearization of amplitude characteristics thereof. One of well-known devices for suppressing or compensating the nonlinear distortion is an adaptive predistortor distortion-compensation device which is used with a power amplifier in a radio transmitter. The power amplifier is hereinafter referred as the amplifier for the sake of simplicity.
In the adaptive predistortor distortion compensator, the nonlinear distortion is decreased with so-called predistortion processing in which the input signal into the amplifier is previously multiplied by compensation factor. The compensation factor has the inverse characteristic based on the nonlinear characteristic in the power amplifier. Since the compensated input signal is amplified by the amplifier having the nonlinear characteristic, the nonlinear characteristic of the amplifier is cancelled by the inverse characteristic in the input signal, nonlinearity in the output signal from the amplifier may be decreased.
Referring to FIG. 6, a device 100 as an example of conventional distortion compensation devices will be explained. The device 100 includes a transmitting system and a feedback system. The transmitting system is configured with a mixer 106, a distortion compensation controller 108, an equalizer 109, a digital-analog converter (DAC) 110, a local oscillator 111, a quadrature modulator (MOD) 112, and a power amplifier (PA) 118. In the device 100, the baseband signal including in-phase and quadrature signals is multiplied by the compensation factor generated with the distortion compensation controller 108 through the mixer 106, where an in-phase signal and a quadrature signal are referred as to I signal and Q signal respectively. Multiplying the input signals by the compensation factor is a so-call predistortion processing.
Then, I and Q signals outputted from the mixer 106 are fed to the equalizer 109 in which frequency characteristics of both signals are compensated. Successively, the I and Q signals are converted into analog signals individually by the DAC 110. With applying the I and Q signals in analog form to the quadrature modulator 112, a radio signal for transmission is generated as described below. In the quadrature modulator 112, a reference carrier signal of a radio frequency and a carrier signal are modulated (quadrature-modulated) with the I and Q signals respectively, and then are added each other to generate the radio signal, where the reference carrier signal and the carrier signal are provided by the local oscillator 111 and the phase of the carrier signal is different by 90° from that of the reference carrier signal. The radio signal is amplified to a specific level by the power amplifier 118 and then radiated into the air through an antenna (not illustrated in FIG. 6).
The feedback system in the device 100 is configured with a delay circuit 103, a calculating unit 105, a directional coupler 132, a local oscillator 133, a down-convertor 134, an analog-digital convertor (ADC) 136, a numerically controlled oscillator (NCO) 137, and a quadrature detector (DEM) 138. A part of the radio signal outputted from the power amplifier 118 is split by the directional coupler 132 and fed to the down-convertor 134 which mixes the radio signal with a signal generated by the local oscillator 133 to convert the radio signal to an intermediate-frequency signal. The intermediate-frequency signal is converted into a corresponding digital signal by the analog-digital converter 136. Subsequently the intermediate-frequency signal in digital form is processed by the quadrature detector 138 for quadrature detection to generate a baseband signal (an in-phase and a quadrature signals) using a signal fed from NCO 137
Hereinafter, a signal flowing in the feedback system is referred as to an FB signal for the sake of simplicity. The delay circuit 103 is arranged for adjusting the phase of the baseband signal (I and Q signals) as the reference signal to coincide with the phase of the FB signal as an output of the quadrature detector 138. The calculating unit 105 calculates the difference value or error between amplitudes of the Ref signal (reference signal) and the FB signal. The difference value is fed to the distortion compensation controller 108 in which the compensation factor allows the difference value to come close to zero. The adaptive predistortion distortion compensation devices such as described above are disclosed in International Publication Pamphlet No. WO2003/103166, Japanese Laid-open Patent Publications No. 2008-78702, 2006-270246, and 2006-279780 for example.